Wireless communication systems are well known and consist of many types including land mobile radio, cellular radiotelephone (inclusive of analog cellular, digital cellular, personal communication systems (PCS) and wideband digital cellular systems), and other communication system types. In cellular radiotelephone communication systems, for example, a number of communication cells are typically comprised of base stations (BS's), generally linked to one or more base station controller(s) (BSC) forming a base station system (BSS). The BSC's are, in turn, linked to a mobile switching center (MSC) which provides a connection between the BSS and a public switched telephone network (PSTN), as well as interconnection to other BSS's. Remote communication units (or mobile stations) (MS) inclusive of all types and styles of moveable/portable radio's, operating within the communication cells, transmit signals to (reverse link) and receive signals from (downlink) the serving BSS. A communication resource, therefore, is a pair of signals comprised of a downlink communication path and a reverse link communication path. The signals are processed by the BS, BSC and MSC to complete a communication transaction with another MS, or through the PSTN to a land-line telephone user.
Fundamental to such wireless communication systems is the ability to maintain established communication connections while moving in and between cell sites. This ability to "handoff" the call between cell sites is a requirement that dates back to the earliest analog wireless communication systems. As technology has improved, the cell sites themselves were split into sectors wherein each sector is serviced by a narrow-beam antenna. In this way, a 360.degree. cell may be split into three 120.degree. sectors, or six 60.degree. sectors. In frequency division multiple access (FDMA) or time-division multiple access (TDMA) wireless communications systems, it is common to assign a set of separate frequencies to each of the sectors, thereby improving the channel reuse pattern of these frequencies among the cellular system from a 12-cell to a 7-cell or a 3-cell channel reuse pattern. As these frequencies represent the spectrum within which an operator is required to operate, its ability to reuse specific channels has a direct affect on its overall system capacity (i.e. the number of MS's the system may handle at a given time) and hence, its profitability. Therefore, it is well-known in the industry to work towards providing a 1-cell reuse pattern as being the optimal wireless communication system from a capacity standpoint.
Increasing capacity through channel reuse comes at a cost, however. A consequence of re-using the same channel in two or more cell's spaced too close together is heightened interference. This interference has a negative impact on the perceived call quality by the end-user, which has a corresponding negative impact on an operator's business. In addition to the interference problems, as the physical area of channel coverage is decreased (from a cell to a sector) the system is required to process increasingly more handoff requests, albeit the majority of these handoffs are intra-BS (from sector to sector within a cell). In FDMA and TDMA systems, wherein the sectors operate on separate frequency channels, the handoff is referred to as a hard-handoff. A hard-handoff is the "term of art" used to signify that the system is actually requiring the MS to change channels to maintain the communication connection. Actually, the communication connection, as it exists on a particular frequency, will be torn-down and reestablished on a separate frequency associated with the channel set of the new BS. This process of tearing-down and reestablishing the communication connection is often perceptible by the end-user and is, at best, an annoyance.
In one type of wireless communication system, the code division, multiple access (CDMA) communication system, a BS communicates with a plurality of MS's via a common wideband communication channel (e.g. 1.25 MHz in IS-95) that uses digital codes to identify the individual MS's within the communication channel. In this way, an operator utilizing CDMA coverage may be able to achieve the optimal 1-cell channel reuse pattern, while maintaining an acceptable level of interference. In addition, because of its inherent ability to use a common channel among a number of sectors within a cell site, the CDMA communication system has introduced the concept of soft-handoff. For ease of explanation, the concept of soft-handoff will be described with reference to the communication system depicted in FIG. 1.
Depicted in FIG. 1 is a CDMA wireless communication system capable of performing soft-handoff. FIG. 1 consists of two adjacent BS's, BS.sub.A 102 and BS.sub.B 106, along with their respective coverage areas 170 and 180. The line 190 represents the direction of travel of MS 104, while the points marked 110, 112, 113 and 114 along line 190 depict decision points for the wireless communication system.
With continued reference to FIG. 1, an ideal soft-handoff scenario begins with a CDMA MS 104 at point 110, where it is being serviced by BS.sub.A 102, driving in a direction marked by line 190. As MS 104 moves towards point 112, the pilot-signal signal strength of BS.sub.B 106 increases until where, at point 112, MS 104 identifies the pilot-signal (not shown) from BS.sub.B 106 as a viable communication resource and requests the wireless communication system 100 to establish BS.sub.B 106 as a diversity communication resource, thereby establishing the radio frequency (RF) communication link 132. At this point, or soon after MS 104 has identified the pilot-signal of BS.sub.B 106, BS.sub.B 106 will attempt to acquire MS 104 on reverse link 134. Reverse link acquisition is said to occur when BS.sub.B 106 can discriminate the unique code used by MS 104 from a plurality of other MS's (not shown) using different codes. Once the reverse link communication link 134 and the forward link communication link 132 have been established, a complete communication resource 130 is said to exist. In an ideal situation, these two events occur near simultaneously at point 112. Thus, where MS 104 is in simultaneous communication with two or more BS's (102,106), MS 104 is said to be in soft-handoff. Therefore, shaded area 150 represents the overlap in coverage of BS.sub.A 102 and BS.sub.B 106 and is referred to as the soft-handoff region 150 between the two BS's.
In FIG. 1, MS 104 continues to travel along path 190 in soft-handoff until it reaches the position marked by point 113. At point 113, MS 104 is nearing the outer bounds of Coverage Area A 170 of BS.sub.A 102. As such, the signal strength of the communication resource 120 is waning and MS 104 determines that BS.sub.A 102 is no longer a useful downlink diversity contributor. As a result, MS 104 requests the communication system 100 to drop the communication resource 120 between BS.sub.A 102 and MS 104. The system signals BS.sub.A 102, in a known fashion, to drop the forward link 122 with MS 104. The result is, at the position marked by point 114, MS 104 has left the soft-handoff region 150 between Coverage Area A 170 and Coverage Area B 180, and MS 104 is no longer in communication with BS.sub.A 102.
As with any ideal scenario, when implemented in real-world conditions there exists obstacles which need to be overcome. Such is the case with implementations of wireless communication systems. One such obstacle which has arisen in the context of CDMA soft-handoff is that of the "orphan condition". Referring again to FIG. 1, the orphan condition arises at the position marked 113 and is characterized as a situation in which the dominating downlink and reverse link are not associated with the same BS. Recall from the discussion above, at position 113, MS 104 has determined that BS.sub.A 102 is no longer a useful downlink diversity contributor and signals the communication system to sever the communication resource 120. In the ideal situation described supra, the communication resource 120 between BS.sub.A 102 will be dropped and MS 104 will be in communication with BS.sub.B 106 via communication resource 130. However, a problem arises when, at position 113, BS.sub.B 106 has yet to acquire MS 104 on its reverse link 134. That is, MS 104 can identify the downlink signal 132 from BS.sub.B 106, yet BS.sub.B 106 cannot identify reverse link signal 134 from MS 104. In such an instance, to sever communication resource 120 between BS.sub.A is tantamount to dropping the call. Thus defines the orphan condition inherent in prior art implementations of CDMA wireless communication systems.
Thus, a need exists to ensure that the original RF link to the first BS is not severed until such time as the communication system may confirm that both the forward and reverse link has been established between the MS and the second BS, thereby mitigating the opportunity for an orphan condition to arise.